1. Field of the Invention
The present invention relates to a wavelength multiplexing unit and a wavelength multiplexing method of a wavelength multiplexing optical transmission system used in optical communications.
2. Description of the Related Art
FIG. 8 is a block diagram showing a wavelength multiplexing unit of a wavelength multiplexing optical transmission system according to a related technique. Explanations will be provided by referring to the drawing.
A wavelength multiplexing unit 90 of a wavelength multiplexing optical transmission system includes: a plurality of transmitters 921 to 92n for outputting signal lights 911 to 91n of different wavelengths from each other; and a wavelength multiplexer 94 for multiplexing the signal lights 911 to 91n outputted from the transmitters 921 to 92n. The transmitters 921 to 92n are connected to ports (connecting ports) 951 to 95n of the wavelength multiplexer 94. The wavelength multiplexer 94 outputs a wavelength multiplexed light 93 by multiplexing the signal lights 911 to 91n. In FIG. 8, λ means a wavelength.
In a general wavelength multiplexing (frequency division multiplexing) optical transmission system, a wavelength (frequency) band is allotted to each channel. Leak light out of the band is a disturbance to other channels, so that it is desired to be avoided as much as possible. The wavelength multiplexing unit 90 of the related technique uses the wavelength multiplexer 94 exhibiting a filter effect in order to avoid emission of a spectrum out of the hand. With such kind of wavelength multiplexer 94, loss is small in the channel band to be passed through whereas loss is large in other bands. Thus, the signals to be passed through can be multiplexed with small loss, and the spectrum of the other bands can be eliminated.
As described, in the wavelength multiplexer 94, the bands of the ports 951 to 95n are fixed to the bands of the signal lights 911 to 91n from the transmitters 921 to 92n on one-on-one basis. That is, the wavelength multiplexer 94 includes a built-in bandpass filter having different bands for each of the ports 951 to 95n. This kind of wavelength multiplexer 94 may sometimes be called as a “wavelength selective type”.
Hereinafter, Patent Documents will be described.
Japanese Unexamined Patent Publication 2008-227556 (Paragraphs 0027-0031, FIG. 2: Patent Document 1) discloses a wavelength multiplexer having a filter characteristic. Further, in Patent Document 1, a word “colorless” is used for indicating that, there are a plurality of wavelengths (though limited) that can be inputted from a given port of the wavelength multiplexer due to the filter characteristic. In the meantime, in this Specification, a word “colorless (i.e., true colorless)” is used to indicate that there is no limit in the wavelengths that can be inputted from a given port of the wavelength multiplexer (i.e., a wavelength multiplexer having no filter characteristic). Furthermore, Patent Document 1 discloses a device called an “interleaver” is used as a 2-to-1 wavelength multiplexer.
Japanese Unexamined Patent Publication 2001-296418 (FIG. 10, FIG. 12: Patent Document 2) discloses about an optical fiber Bragg grating (FBG).
Japanese Unexamined Patent Publication 2010-054624 (FIG. 7, FIG. 9, FIG. 22, FIG. 24, FIG. 34: Patent Document 3) discloses a wavelength tunable filter which heats AWG (Arrayed Waveguide Grating), Mach-Zehnder interferometer, or a ring resonator with a heater.
Japanese Patent No. 2874439 (FIG. 1: Patent Document 4) discloses a wavelength tunable filter using a dielectric multilayer film.
Japanese Unexamined Patent Publication 2002-122747 (Paragraph 0066, FIG. 1: Patent Document 5) discloses a grating component in which a plurality of kinds of gratings of different reflection wavelengths are formed in series in the length direction of optical fibers.
In a case where the transmitters 921 to 92n are of the wavelength fixed type, the number of models thereof becomes enormous. Thus, management thereof becomes complicated. In the meantime, when the transmitters 921 to 92n are of the wavelength tunable type, the variation of models can be reduced since the output wavelength can be set arbitrarily according to commands. Therefore, it is possible to overcome the issue of managing the variations.
However, even though the wavelengths of the wavelength tunable type transmitters 921 to 92n can be changed remotely, connections between the transmitters 921 to 92n and the wavelength multiplexer 94 cannot be remotely switched. Therefore, an operator needs to switch the connections manually (see a virtual line 96). This is because the ports 951 to 95n and the passing wavelengths are fixedly related in the wavelength multiplexer 94.
In order to overcome such issue, there is considered a technique which switches connections by using a large-scaled optical matrix switch. However, this technique is unpractical since the structure of the large-scaled optical matrix switch is complicated and costly even though the wavelength multiplexer 94 can be utilized as it is.
It is therefore an exemplary object of the present invention to provide a wavelength multiplexing unit and a wavelength multiplexing method of a wavelength multiplexing optical transmission system which can connect a wavelength tunable transmitter and a wavelength multiplexer remotely with a simple structure, when changing the wavelength of the wavelength tunable transmitter remotely.